This invention relates to the control of skipped-ratio power-on downshifting in an automatic transmission that is optimized for sequential ratio shifting.
In general, a motor vehicle automatic transmission includes a number of gear elements and selectively engageable friction elements (referred to herein as clutches) that are controlled to establish one of several forward speed ratios between the transmission input and output shafts. The input shaft is coupled to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gearset. Shifting from a currently established speed ratio to a numerically adjacent speed ratio involves, in most cases, disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio.
Since most shifts involve numerically adjacent speed ratios (i.e., sequential shifting), the fluid control hardware can be designed to minimize the number of modulated pressure control valves, as disclosed for example, in the U.S. Pat. No. 5,601,506 to Long et al., issued on Feb. 11, 1997, and assigned to the assignee of the present invention. In Long et al., a set of relatively inexpensive on/off relay valves selectively couple the various transmission clutches to two modulated valves, such that a certain combination of clutches can only be coupled to a given modulated valve, and for any shift to a numerically adjacent speed ratio, one of the modulated valves is coupled to the on-coming clutch, and the other modulated valve is coupled to the off-going clutch. While such an arrangement can significantly simplify the control hardware and reduce manufacturing costs, it essentially rules out skip-shiftingxe2x80x94that is, shifting to speed ratio other than a numerically adjacent speed ratio. Thus, if the transmission is operating in third gear, for example, and the engine load abruptly increases to a level for which first gear would be appropriate, the controller must successively perform sequential shifts from third-to-second, and from second-to-first, instead of skip-shifting from third-to-first. Accordingly, what is needed is a control methodology for performing skip-downshifts in a transmission where the control valve configuration is optimized for sequential shifting.
The present invention is directed to an improved downshift control for an automatic transmission optimized for sequential shifting, wherein skip-downshifting to a target gear is achieved by shifting the transmission to neutral and controlling the engine output torque while the transmission is reconfigured to establish the target speed ratio. The engine torque control is designed to allow the transmission input speed to reach the synchronization speed for the target speed ratio at a desired time, and the shift is completed when the transmission input speed actually reaches the synchronization speed. The engine torque control is predicated on a modeled engine torque parameter, and the torque control includes an adaptive parameter for adaptively adjusting the torque control to ensure that the input speed reaches the synchronization speed at the desired time.